In catalytic converters having a monolith with a plurality of catalyst coated passages through which internal combustion engine exhaust gases are directed for treatment, the gas velocity in the converter's inlet snorkel is normally nonuniform thereacross. For example, the velocity profile typically resembles a parabolic curve with the velocity maximum at the center and decreasing significantly out toward the monolith periphery. As a result, the exhaust gases are unevenly distributed between the monolith passages and not all the catalyst is effectively utilized.
Various concepts have been proposed to improve flow distribution to the monolith passages and thus to the catalyst. Typically, these designs have attempted to alter the exhaust gas flow in the inlet snorkel before it reaches the upstream face of the monolith. The two most common types are diffusers and modified inlet snorkels. The diffusers are normally mounted upstream of the monolith and designed to create very turbulent flow within the snorkel for improved distribution. On the other hand, the modified or special inlet snorkel designs typically include large diameter inlet pipes, smaller diverging angles, dual pipes or some combination thereof. Both diffusion and snorkeling require the flow distribution to become uniform whether the entering flow is in laminar or turbulent form. This is particularly difficult to do with a diffuser. Moreover, an upstream diffuser extracts heat thereby increasing the catalyst warm-up time. Snorkels also add to the warm-up time as well as present an additional problem of space allocation since lessening the diverging angle to improve the flow distribution adds to the length of the snorkel and thus to the heat extraction prior to monolith entry.